Fluid induced melting in mantle xenoliths and some implications for the continental lithospheric mantle from the Minusinsk region (Khakasia, southern Siberia)

Eleven representative xenoliths from the Minnsinsk Region, southern Russia were studied in order to highlight the characteristic features of the subcontinental lithospheric mantle beneath the region. Type-I xenoliths show that the lithosphere underwent various degree of depletion overprinted by enrichment processes leading to LREE-enriched pyroxenes. Estimated equilibrium temperature for the xenoliths is in the range of 960-1050°C. Type-II xenoliths are the result of crystallization from a possibly basaltic melt close to the crust-mantle boundary. Three xenoliths in the Type-I series show evidence of incipient melting such as spongy rims of pyroxenes and interstitial glass. The spongy rim of clinopyroxene consist of clinopyroxene and glass with modal proportion of approximately 82 and 18%, respectively. Orthopyroxene rim contains olivine (65%) and glass (35%) with subordinate amounts of clinopyroxene (<5%). Glass within the spongy rim exhibits a clear geochemical affinity to interstitial glass as both have similarly high Al₂O₃, SiO₂ and alkali contents. The interstitial glass and the spongy rims (minerals + glass) display light rare earth (LRE) element and large ion lithophile (LIL) element enriched character. This indicates that incipient melting of pyroxenes occurred in an open system and was likely triggered by the influx of a Na alkali silicate melt/fluid. The interstitial glass represents the residual melt after interaction with the pyroxenes. The formation of this Na-rich silicate melt may represent an earlier stage of the mantle magmatic event that produced the host basalt.